The technologies described in Patent Literatures 1 to 3 below are known as technologies about two-cycle engines. In the engine described in Patent Literature 1, a side face of a piston is provided with a belt-like groove that establishes communication between an exhaust port and a scavenging port only when the piston is located at a specific position where the exhaust port and the scavenging port are closed. This causes part of burnt gas to be taken as EGR (Exhaust Gas Recirculation) gas into the scavenging port and to be circulated into a combustion chamber at earlier timing than fresh mixture gas. As a result, combustion efficiency is improved and THC (Total HydroCarbons) in exhaust gas is reduced.
The engine described in Patent Literature 2 adopts the stratified scavenging using four-flow Schnuerle scavenging as a scavenging system. The Schnuerle scavenging is a scavenging method of letting scavenging flows out of opposed scavenging ports collide with each other to form reverse swirls and scavenging the interior of the combustion chamber by the reverse swirls. Specifically, a pair of scavenging ports located on the exhaust port side of the cylinder are provided for EGR gas and a pair of scavenging ports located on the suction port side of the cylinder are provided for fresh mixture gas. Four scavenging passages scavenging directions of which are precisely determined function as guide passages for injecting the fresh mixture gas and EGR gas into arbitrary directions in the combustion chamber in the scavenging stroke and function as partitions for keeping a fresh mixture gas layer and an EGR gas layer from mixing together. The engine is configured so that the EGR gas sweeps past into the exhaust port after scavenging of the combustion chamber while the fresh mixture gas stays in the combustion chamber without sweeping past.
In the engine described in Patent Literature 3, the center of a spherical surface of a piston crown is offset by a predetermined lateral length from the center axis of the piston. This configuration provides the spherical surface of the crown with deflector-like effect and guide action for reversal of scavenging flow in the combustion chamber, thereby improving scavenging efficiency. (Patent Literature 1: Japanese Unexamined Patent Publication No. H11-287124, Patent Literature 2: Japanese Unexamined Patent Publication No. 2001-140651, Patent Literature 3: Japanese Examined Utility Model Publication No. H3-52981)